Commercially manufactured solid state motor stacks, or actuators, are produced using piezoelectric discs interleaved with metal foil electrodes. Application of electrical potential applied to alternately biased electrodes causes each of the piezoelectric discs to expand or axially distort. The additive deflection of the stacked discs is typically amplified by hydraulics to effectuate useful actuation.
An example of a conventional electromechanical actuator having an active element of electroexpansive material is found in U.S. Pat. No. 3,501,099 and 3,635,016 to Glendon M. Benson. Benson's patents disclose both an actuation amplification structure and a method for manufacturing piezoelectric stacks. Sheets of ceramic material are rolled, compacted and punched into ceramic discs. After a cleaning process, the discs are stacked with alternate sets of continuous disc electrodes disposed between the ceramic discs. The stacks undergo a pressurized cold-welding process, followed by an elevated temperature and pressure bonding process after common electrodes are connected to the two electrode groups. The stacks are poled by application of a DC voltage and then encapsulated with a plastic insulative cover prior to final mounting within a transducer housing.
Various environmental design considerations are important in piezoelectric stack manufacturing. Device operating temperature ranges and external mechanical stresses are the most serious of these factors.
Conventional stacks are limited to a maximum operating temperature of about 75.degree. Celsius, measured at the outside of the stack housing. Heat generated by the stack itself is compounded by the ambient temperature. For example, one application of the stack may be to actuate the valves on an engine. Extreme heat generated by the engine upon which the housed stack is typically mounted typically results. Stack temperatures can reach upward of 40.degree.-50.degree. C. above the measured engine temperature.
On the other hand, structural defects typically lead to conventional stack failure due to shear and torsional stresses applied to the stack during operation and/or installation. Structural stack failure is most commonly attributed to fatigue cracking of the ceramic discs. Separation between disks/electrodes is also a frequent problem.
Piezoelectric stack insulation has been introduced between the disk/electrode stack and the housing in an attempt to minimize some of the above mentioned problems.
U.S. Pat. No. 4,011,474 to Cormac G. O'Neill discloses several methods for improving stack insulation to avoid operation breakdowns. Arc-over is allegedly avoided by maintaining contact between the piezoelectric stack and the insulating material. In a first embodiment, O'Neill teaches introducing a pressurized insulating fluid such as oil, into the housing of a piezoelectric stack. The fluid is pressurized so as to maintain contact between the fluid and the stack during radial shrinkage, or axial expansion, upon the application of an applied voltage.
In a second embodiment, O'Neill applies a solid polyurethane coating to the stack. The coating is kept in contact with the stack by a pressurized insulating fluid to prevent separation during operation and arc-over associated therewith.
A third O'Neill embodiment maintains contact between the stack and a solid insulating coating by winding a filament or tape around the coated stack. The tape is wound around the coating to preload the coating to prevent separation of the coating from the stack. The winding of the tape is spaced to allow for expansion of the polyurethane coating during operation of the stack.
The present invention constitutes an improvement over conventional encapsulation technology. Benefits, such as increased stack operational temperature range, endurance, output, and lifetime, are achieved by the present invention.